After takeoff from a canal in a float plane, while cruising about 500 feet above a swamp, the engine quit. The airplane was force landed in the swamp. Following the accident, the pilot examined the engine and found water in the gascolator and the fuel line between the gascolator and the carburetor. He also reported that the airplane was equipped with long range fuel tanks that had flush fitting fuel filler caps. Previously, he had had problems with water in the fuel.